Harbor Police spokesman Matthew Gresham said the man, identified as John Styen of Allegheny, Pa., was rescued by one of the service's 25-foot small boats shortly before. Styen was treated for chills by EMS responders, then transferred to Interim LSU Hospital for "supervision and evaluation," Gresham said.

A passerby alerted Harbor Police that a man was in the water at 4:23 p.m., Gresham said. The service's fire boat and two Coast Guard vessels were dispatched and arrived within minutes.

The man initially tried to swim away, police said, but he was corralled by the boats and hauled aboard at 4:52 p.m., Gresham said. Styen was brought aboard the Coast Guard boat about midway between the Governor Nicholls Street Wharf and the Moonwalk, Gresham said.

Jeff Mohnkern, a tour guide with Royal Carriages, said he left his staging area at Jackson Square to see what the commotion was about after police scrambled toward the river near the Jax Brewery building. Mohnkern said the man already had been fished out of the water and taken from view.

Mohnkern's co-worker Brett Crawford was one of the first to spread the word on the incident via Twitter:

Some guy just jumped on the Mississippi and is trying to swim to Algiers. Coast Guard trying to get him. Crazy and dumb.

Attempting to swim across the river can bring a range of charges, from disturbing the peace to felonies of simple obstruction of a highway of commerce (a navigable waterway, punishable with up to six months in jail), to aggravated obstruction of the waterway, which could result in a prison sentence of up to 15 years.

As of Monday evening, however, Gresham said it appeared Styen would not be charged for his aquatic adventure.